XR80 Engine Build

vintagemx · Dec 3, 2012

Okay a while back I picked up a couple XR80 engines. One was in running condition and the other in unknown condition. Since these are still legal for the XR75 class I dropped the runner in a '74 XR75. Now that The Project (a replica '77-'78 XR75) is ready for an engine I pulled the unknown out. Had a few parts in bags including 2 pieces which, together, make half the cam gear. Pulled the valve cover and sure enough no cam gear. In fact the lower cam bearings are wiped out. Thinking the other half of the cam gear is in the bottom end.

So now it's time to tear down the engine and figure out what all it needs. Would like to do a big bore kit, but want to keep the bike legal for the XR75 class. So I'll stay with the stock or 2mm max overbore for now. Working on sourcing a head and I'll need a new cam too. Probably go with an XR100 carb since I have the XR100 air box connector.

Feel free to throw out any thoughts.

storminnorman07 · Dec 3, 2012

whats CC limit, what racing app, and how crazy do you wanna get?

just throwing some ideas out there

-over size the intake to 26mm
-web cam #68
-valve spring kit
-press crank apart and put that gal on a diet. depending on slug size desired longer rod is best if pressing crank apart to open piston options up.
-some port work
-lighten flywheel

or

cheat like hell?

Brodie · Dec 3, 2012

opcorn:

Luckystar64 · Dec 3, 2012

Are there any roller cam heads available for an XR80/100 that would work with the stock bore? --L*64

vintagemx · Dec 3, 2012

Took the top end off last night. Has a fresh 48mm piston (0.5mm oversize). No scuffing/scoring on the piston skirt or bore. Crosshatch is still visible. Zero carbon build up on the piston crown. Only damage I can see is cooked cam bearings in the head and scored cam journals. Looks like it was a fresh rebuild. Need to check out the oil pump next.

vintagemx · Dec 3, 2012

storminnorman07 said:
whats CC limit, what racing app, and how crazy do you wanna get?

or cheat like hell?

Vintage motocross, XR75 class for 8-12 year olds on stock XR75 or XR80 machines. Not looking to get too crazy. Maybe some porting, a cam and bigger carb. Keep it close to stock. For the 2 seasons he'll race it.

storminnorman07 · Dec 3, 2012

vintagemx said:
Vintage motocross, XR75 class for 8-12 year olds on stock XR75 or XR80 machines. Not looking to get too crazy. Maybe some porting, a cam and bigger carb. Keep it close to stock. For the 2 seasons he'll race it.

got ya. i agree if your son will only be using it for 2 seasons no need to get crazy with it unless he is way under horsed in respect to the other racers. lil porting, put a tb or web #68 cam in it, bigger carb, if your comfortable with degreeing cams lop off 0.020" on head to give a bump in CR. i STRONGLY urge since you have it gutted to throw a HV oil pump on it and put a shower cap on it. i know the shower cap is a pretty pricey piece. but really helps aid in not cooking cam journals along side w/ the HV pump. The xr 80 has a smaller oil pump than the xr 100. so just by putting a xr 100 oil pump on it you'll pick up a lil more flow. but would suggest nutting up and getting the takegawa hv pump.

vintagemx · Dec 3, 2012

storminnorman07 said:
got ya. i agree if your son will only be using it for 2 seasons no need to get crazy with it unless he is way under horsed in respect to the other racers. lil porting, put a tb or web #68 cam in it, bigger carb, if your comfortable with degreeing cams lop off 0.020" on head to give a bump in CR. i STRONGLY urge since you have it gutted to throw a HV oil pump on it and put a shower cap on it. i know the shower cap is a pretty pricey piece. but really helps aid in not cooking cam journals along side w/ the HV pump. The xr 80 has a smaller oil pump than the xr 100. so just by putting a xr 100 oil pump on it you'll pick up a lil more flow. but would suggest nutting up and getting the takegawa hv pump.

We also do some trail riding so making it a full-blown racer isn't the best option. Need to get the side cover off and check out the oil pump next.

theraymondguy · Dec 3, 2012

XR100 Carb will require xr100 intake. XR100 cam adds a bit of lift over stock. XR100 Clutch has 1 more disc than XR80. Stock flywheel is hefty, you can have it lightened about .75 Lb. Any of the aftermarket heads I've seen are all made for big bore kits, whether they'd work on a stock bore I can't say - though Chad would know for sure. Late model heads (92 and up) have larger cooling fins.

storminnorman07 · Dec 4, 2012

vintagemx said:
We also do some trail riding so making it a full-blown racer isn't the best option. Need to get the side cover off and check out the oil pump next.

lol these ole tail draggers even with some pretty hefty work they are still great as woods pounders. pretty general mod work that myself and terry at FPM do for the 80's is... 57 slug, lighten crank and put a long rod on it to do away with the ridiculous compression height of the factory setup, lil port work, tb cam or web cam grind #68 or 68/69, 24mm carb, HV oil pump, and either lighten the factory flywheel or do a retro fit outer rotor

storminnorman07 · Dec 4, 2012

theraymondguy said:
XR100 Carb will require xr100 intake. XR100 cam adds a bit of lift over stock. XR100 Clutch has 1 more disc than XR80. Stock flywheel is hefty, you can have it lightened about .75 Lb. Any of the aftermarket heads I've seen are all made for big bore kits, whether they'd work on a stock bore I can't say - though Chad would know for sure. Late model heads (92 and up) have larger cooling fins.

the aftermarket heads "could" work w/ a smaller bore than what they are kitted with. problem with it is 1.) ports and cams are more suited for the BB application 2.) finding a dome to bring compression where it needs to be with the aftermarket head and factory/close to factory bore will prove to be difficult.

vintagemx · Dec 4, 2012

Ok must've been a bean counter who dictated using phillips head screws for the side covers. More than half are mangled. Was able to get 3 loose with my impact driver. Gonna have to drill the heads off the rest.

Luckystar64 · Dec 4, 2012

Damn. 8 to 12 year olds? Why should they get to have all the fun? I'd LOVE to run an XR124 powered XR75! You guys are ignoring my question about a roller cam head for the stock motor. I get annoyed with the way the cam rides on the bare aluminum in an XR100 motor. --L*64

storminnorman07 · Dec 4, 2012

Luckystar64 said:
Damn. 8 to 12 year olds? Why should they get to have all the fun? I'd LOVE to run an XR124 powered XR75! You guys are ignoring my question about a roller cam head for the stock motor. I get annoyed with the way the cam rides on the bare aluminum in an XR100 motor. --L*64

Are you meaning just make cam a ball/needle bearing. Or convert Stocker head to a roller head with ball/needle bearings?

vintagemx · Dec 4, 2012

I think he's talking about a roller bearing for the cam bearing journals. The Honda design is cheap, but poor. A steel cam journal with the lower (loaded) half of the cam bearing machined in the head and the upper (unloaded) half in the cast iron rocker holder.

Luckystar64 · Dec 4, 2012

Actually-- I was looking for an aftermarket haed that uses a roller cam. The stock setup is junk. --L*64

vintagemx · Dec 5, 2012

Luckystar64 said:
Actually-- I was looking for an aftermarket haed that uses a roller cam. The stock setup is junk. --L*64

A roller cam, which I assume you mean using roller rocker arms, would be sweet... And expensive. Not sure how that would cure the "design flaw" of the cam bearing machined into the cylinder head though. The early ('73-'76) XR75's had a much better design with the cam bearings in a separate piece that bolted onto the cylinder head.

storminnorman07 · Dec 5, 2012

Luckystar64 said:
Actually-- I was looking for an aftermarket haed that uses a roller cam. The stock setup is junk. --L*64

i got ya now. yeah takie has had this covered for a while now. they first brought out a super head (non roller rockers) with a ball bearing cam. which was short lived and replaced with the +R. here are a couple pics of the +R. i at one point in time had a thread on here showing the difference between the takegawa stage 1, stage 2, stage 3, +R and kitacos se for xr 100. but a while back a few members pissed me off and i went on a deleteing spree. so this was all i could find through my post of it.

one of my failed attemps on a 73'-76' xr 75

Luckystar64 · Dec 5, 2012

So Takegawa has a roller cam/roller rocker head that would work with the stock motor? I don't want to get the whole Tak 99cc jit. I'd rather just have a decent head to bolt on the stock motor. The racing class I was looking to race the bike in has a 100cc max. --L*64

storminnorman07 · Dec 6, 2012

Luckystar64 said:
So Takegawa has a roller cam/roller rocker head that would work with the stock motor? I don't want to get the whole Tak 99cc jit. I'd rather just have a decent head to bolt on the stock motor. The racing class I was looking to race the bike in has a 100cc max. --L*64

no the +r kits start off at 115cc and go up to 146 or 148cc. you "could" techinically bolt a +r head up to the stock engine but you would have to source out a domed piston. if one were to just place the +r head upon the stock bascially flat top piston you wouldn't have chit for compression.

XR80 Engine Build

vintagemx

storminnorman07

Brodie

Luckystar64

vintagemx

Attachments

vintagemx

storminnorman07

vintagemx

theraymondguy

storminnorman07

storminnorman07

vintagemx

Luckystar64

storminnorman07

vintagemx

Luckystar64

vintagemx

storminnorman07

Luckystar64

storminnorman07

Top Contributors this Month

Recommended Communities